Optical harmonic generation may be used to convert laser light from one wavelength to a shorter wavelength (i.e., a higher frequency). For example, frequency doubling, or second harmonic generation (SHG), may be used to obtain visible light from near infrared light. The frequency doubled light may then be used for generating frequency tripled light, spectroscopy, materials processing, optical pumping, and/or the like.
Typically, the optical frequency of laser light is doubled using a nonlinear optical that uses a single input beam to provide a pair of photons required for conversion to the second harmonic. FIG. 1 is a diagram illustrating a prior art second harmonic generator 10. As shown in FIG. 1, prior art second harmonic generator 10 includes second harmonic crystal 14. In operation, fundamental optical beam 11 at optical frequency ω impinges on second harmonic 14. Since a nonlinear conversion efficiency of second harmonic crystal 14 is less than 100%, only a portion of fundamental optical beam 11 is frequency doubled within second harmonic crystal 14, so that second harmonic beam 12 at a second harmonic frequency 2ω exits second harmonic crystal 14 together with an unconverted portion 11A of fundamental optical beam 11 at optical frequency ω. Thus, two beams exit second harmonic 14: unconverted portion 11A of fundamental optical beam 11 and second harmonic beam 12. In some applications, second harmonic beam 12 and unconverted portion 11A may impinge on a third harmonic crystal (not shown), which converts a portion of these beams into a third harmonic beam at a third harmonic frequency. Alternatively, in some cases, second harmonic beam 12 may be propagated for another purpose. For example, prior art second harmonic generator 10 may include a dichroic mirror (or filter) that redirects unconverted portion 11A and transmits second harmonic beam 12 as an output.